Cutting Glass Tubing

[Sean TaylorParticipant @seantaylor98558]

[Sean TaylorParticipant @seantaylor98558]

Hi all,
I'm new here, and have been browsing, and I noticed a few posts on the subject of cutting glass tubing. This is how it's done by glassblowers:

• Score with tungsten carbide, or diamond – the score should be on the line of cut, and min. 1/8 circumference – longer is better
• wet score with water (saliva works fine) to prevent 'healing'
• touch small, hot end of glass rod to one end of score
• wait, applying light pressure. the score will crack and run around the tube
• if tubing doesn't separate on its own, apply axial tension, with very slight bend away from crack (in the 'opening' sense)
• Voila! The cut edge will need grinding/fire polishing to finish.

Small diameters (up to about 10 – 12mm) can be cut by scoring and pull/bending, without the need for heat, as long as they are long enough to get a good grip. Otherwise, heat is normally applied to the score by heating the end of a small dia. glass rod to molten and white-hot (oxy/propane, although for very small dia. rod, propane alone will get there) and transferring quickly to one end of the score line. Soldering irons aren't hot enough, I wouldn't think, neither burning meths, although a petroleum lighter fluid might be worth a try. Also worth trying would be a small blowtorch – larger dia. tubing is often cut using a long score and a very small oxy/propane flame while the work is rotated in a glass lathe.

Hope this is of some use – happy to answer questions if you have 'em.

Sean

(Glassblower, of sorts.)

[jason udallParticipant @jasonudall57142]

Fortune favors the brave
In this case bold…
Always had most success in bold cuts..hesitation seems to guarantee failure

[MuzzerParticipant @muzzer]

Interesting insight, thanks for posting. What is the reference to "healing"? Does the score not happen correctly if it's dry? Sounds like one of those handy tips you'd never work out yourself!

[Sean TaylorParticipant @seantaylor98558]

The score in the glass is edged with microfractures which quite rapidly close and 'heal' if left open to the air. Wetting prevents this closure, and allows thermal expansion to complete the cut. I have found that scoring with a wet glass knife makes for an easier cut than scoring dry and wetting immediately afterward – so I suspect this 'healing' effect can be very quick.
There's a wee tutorial with pics Here, which shows several cutting methods. It suggests using a file; don't, it makes a horrid wide graze as opposed to a score line, unless very sharp. Glass knives are expensive and hard to come by, but B&Q sell a very cheap hand-held tile cutter with a very short, square-section carbide blade which I have used successfully many times (because I am, at heart, a miserly skinflint).

HTH,

Sean

[Douglas JohnstonParticipant @douglasjohnston98463]

I remember cutting an old piece of glass for my workshop window many years ago and having very little success. I was then told that old glass does not cut well for some reason. Was this just an urban myth put about by glass merchants or is it true?

Doug

[Gordon WParticipant @gordonw]

Doug- yes old glass is difficult , but I don't know why. Glass also flows, old window glass is thicker at the bottom.

[Sean TaylorParticipant @seantaylor98558]

Don't know about old glass – the thing about glass flowing isn't true, though, I'm afraid. Old window glass was made by dipping from a furnace and spinning out on the end of an iron – the windows were cut radially from the disc, which was thicker at the centre than the edges. Glaziers wisely put the thicker, heavier part at the bottom of the window.

[Tim StevensParticipant @timstevens64731]

The problem with old glass applies equally to used window or picture glass, which rarely cut sensibly. It is because old glass, unlike new, is covered with a myriad of tiny scratches which in effect act as 'stress raisers' and so any intentional crack can be led astray.

In chemistry labs in the 1960s, use was made of a device similar to the small plumbers pipe cutter, with a hard roller in a form of clamp. In this case the roller was of carbide, so a small groove was produced around the glass tube, which would then snap as required with a minor side load. And it did not seem to work only on 'new' tubing. I have also seen use made of a triangular 'file' of carbide (but without file teeth), so I guess that a modern diamond coated needle file would serve, too. It can help reduce problems if the cut end(s) are heated until the glass starts to soften, as this will close up any incipient cracks, and also makes the rubber tube slide on easier.

Regards, Tim

[Paul LousickParticipant @paullousick59116]

We recently cut 1/2" dia glass tubing for a gauge glass by wrapping the cut area with masking tape and then cutting it with a 1mm wide blade in an angle grinder. Cuts easily.

Paul.

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